This invention relates to methods of separating polymers from solutions.
High molecular weight, elastic polymers find extensive usage in industry. Almost a billion pounds of synthetic rubbers, such as polybutadiene, are used yearly in the American automobile industry alone. Other synthetic elastomers, such as ethylene copolymers, butyl-rubbers and polyisoprene are used widely also in machinery, floor coverings and household goods.
The manufacture of some elastic polymers typically requires large amounts of energy and is carried out in solution with a solvent and various catalysts. For example, in manufacturing polybutadiene, a butadiene monomer is dissolved in commercial grade n-hexane (approximately 45% to 65% n-hexane with methylpentane, dimethylbutane and other organic residuals). In order to obtain a rubbery polybutadiene, organo-metallic and other catalysts are used to produce the cis-isomer rather than the trans-isomer. The polymerization is conducted at elevated temperatuares and terminated by the addition of stoppers. When the reaction is completed the solvent is typically evaporated and recycled. The evaporation step is expensive in terms of energy used requiring over 5000 BTU per lb. of synthetic rubber which is produced.
Other elastomers are produced in similar fashions. Ethylene copolymers are formed by copolymerizing ethylene with at least one non-conjugated diene, and preferably with other .alpha.-monoolefins such as propylene, as well. These copolymers are also formed in solutions of solvents such as tetrachloroethylene, or simple hydrocarbons such as pentane, hexane or heptane; the same problem of separating the polymer and solvent occurs in these systems. Likewise, elastomers such as polyisoprene are typically formed from isoprene monomer polymerized in hexane solutions. The product and the solvent are also separated by energy-intensive evaporative methods.
Attempts have been made to eliminate the solvent evaporation step in polymer production. For example, U.S. Pat. No. 3,553,156, issued to Anolick on Jan. 5, 1971, points out that ethylene copolymer solutions can exist as two phase systems and suggests that temperature and pressure can be varied to arrive at the two phase region. The two phase phenomenon in polymer-solvent systems is a result of the partial miscibility of the constituents. At the normal operating temperature and presure of polymer reactors the constituents exist as a single phase of uniform concentration, C.sub.o. However, when subjected to sufficient heating or pressure changes, the system will break up into two phases at concentrations C' and C". Physically the solution becomes cloudy at the phase boundary due to the difference in the refractive indices of the two solutions.
At the concentrations encountered in industrial polymer reactions, phase-separation typically occurs as a nucleation and growth phenomenon of the solvent-rich phase. Although the method described by the Anolick patent can produce phase-separation, at industrial concentrations the resulting two-phase system exists as tiny bubbles of the solvent-rich phase trapped in an encapsulating, viscous, polymer-rich, phase. In such systems and especially for the industrially important high molecular weight polymers, the physical separation of the phases is extremely difficult and time-consuming.
Therefore, there exists a need for a low-energy method of separating polymers from solvents in high-concentration, industrial manufacturing solutions.